I don't know a huge amount about Cap's rogue gallery so I may be off base here but, wouldn't Lukin be the most logical option? Everything about the Winter Soldier is steeped in cold war paranoia and myth making, so wouldn't it make the most sense to play up the Russian angle? Zola or Zemo are both affiliated with Germany or the Nazis, I think, so it would be something of a missed opportunity and squandering of potential to have them pulling the strings when they're not entrenched in that kind of history.